The present invention relates to a method for recovery of aqueous wash in a phosphate chemical conversion and an apparatus for metal surface treatment.
The phosphate chemical conversion has been frequently used in the pretreatment of shaped metal products prior to coating. In this phosphate chemical conversion, the shaped metal product must be cleaned with water after the chemical conversion treatment. This cleaning involves multi-stage washing with water and in the final stage of cleaning, fresh aqueous wash is used. The overflow of this water is recycled serially to the preceding stages and a portion of the washes from the first stage is discharged from the system, whereby the contaminant concentration of water in each stage is controlled so as to maintain a steady chemical conversion treatment. The aqueous wash from the first stage contain metal ions such as zinc, nickel and manganese ions, as well as ingredients of the phosphate chemical conversion such as phosphate ions, nitrate ions, hydrofluoric acid, hydrosilicofluoric acid, fluoroboric acid, etc., which, if discharged as they are, cause pollution of river and other water. Therefore, it is common practice to pool these washes with other plant effluents and subject the pooled water to flocculation-sedimentation or biological treatment before disposal.
Referring to the aqueous wash produced in such a phosphate chemical conversion, various methods utilizing reverse osmosis membranes for recovery of useful components and for reducing the amount of effluents have been reported. In order to improve the rate of recovery of useful components by a reverse osmosis technique, it already belongs to the known technology to install two reverse osmosis membrane modules in series so that the concentrated water produced in the first module is further treated in the second module to give a concentrated water and a filtrate. However, when the aqueous wash to be treated contains substances which will form precipitates on the membrane-water interface, such as metal salts, chances for precipitation of such substances on the membrane surface of the second reverse osmosis module are high so that the membrane flux of the second reverse osmosis module drops gradually. The resultant disadvantage is that the equipment cannot be operated on a steady basis for many hours.
On the other hand, it is well known that in order to further improve the quality of the filtrate, the filtrate may be recycled to the first reverse osmosis module to thereby reduce the ion concentration of the aqueous wash to the first reverse osmosis module. Furthermore, Japanese Kokai Publication Hei-9-206749 discloses a method which comprises supplying an antiscaling agent to the water to be treated and adding an acid to the concentrate obtained in the first module before feeding it to the second reverse osmosis module. However, this method is disadvantageous in that a filtrate of good quality cannot be obtained, for instance. In addition, neither of these methods offers a solution to the problem of said build-up of precipitates on the membrane-fluid interface.
The object of the present invention is to provide a method and an apparatus for efficient recovery of a useful component and production of a filtrate of improved quality by means of reverse osmosis membranes from the aqueous wash produced in a phosphate process for surface chemical conversion of a shaped metal product.
The present invention is relates to a method for recovery of aqueous wash in a phosphate chemical conversion of a shaped metal product involving carrying out chemical conversion and subsequent cleaning with water,
wherein said cleaning with water is performed in one or more stages and comprises
a step of withdrawing aqueous wash from a first cleaning stage and adjusting the pH of the wash with at least one acid selected from the group consisting of phosphoric acid, nitric acid, hydrofluoric acid, hydrosilicofluoric acid and fluoroboric acid,
a step of treating said pH-adjusted aqueous wash with a first reverse osmosis membrane to separate it into a first filtrate and a first concentrate, and
a step of neutralizing said first filtrate with an alkali and treating the alkali-neutralized filtrate with a second reverse osmosis membrane to separate it into a second filtrate and a second concentrate,
said first concentrate being recycled for said phosphate chemical conversion,
said second filtrate being recycled as aqueous wash for said aqueous cleaning, and
said second concentrate being discarded from the system.
In said pH adjusting step, phosphoric acid may be used as the acid and the pH is adjusted to 2.0 to 3.0.
The apparatus for metal surface treatment according to the present invention is for use in a phosphate chemical conversion of a shaped metal product, which comprises
a phosphate chemical conversion means,
a means for performing aqueous cleaning in one or more stages,
a means for withdrawing aqueous wash from a first stage of said aqueous cleaning means and adjusting the pH of the aqueous wash with an acid selected from the group consisting of phosphoric acid, nitric acid, hydrofluoric acid, hydrosilicofluoric acid and fluoroboric acid,
a first reverse osmosis membrane module for treating the pH-adjusted aqueous wash,
a means for alkaline neutralization of a filtrate from said first reverse osmosis membrane module, and
a second reverse osmosis membrane module for treating the alkaline-neutralized filtrate.
In said apparatus for metal surface treatment, a concentrate from said first reverse osmosis membrane module may be recycled for said phosphate chemical conversion
and a filtrate from said second reverse osmosis membrane module may be recycled as aqueous wash for aqueous cleaning.
The aqueous wash mentioned just above may be from the last stage of aqueous cleaning.